In a photo-electron spectrometer of hemispherical analyzer type, a central component is the measurement region in which the energies of the electrons are analysed. The measurement region is formed by two concentric hemispheres, mounted on a base plate, and with an electrostatic field applied between them. The electrons enter the measurement region through an entrance and electrons entering the region between the hemispheres with a direction close to perpendicular to the base plate are deflected by the electrostatic field, and those electrons having a kinetic energy within a certain range defined by the deflecting field will reach a detector arrangement after having travelled through a half circle. In a typical instrument, the electrons are transported from their source (typically a sample that emits electrons after excitation with photons, electrons or other particles) to the entrance of the hemispheres by an electrostatic lens system comprising a plurality of lenses having a common and substantially straight optical axis.
The lens system and the detector arrangement will only accept electrons which are emitted within a limited area perpendicular to the lens axis and within a limited angular range. Furthermore, the source has to be positioned within a narrow range in the z-direction to achieve the best properties (in terms of sensitivity and resolution). This necessitates mounting the sample on a manipulator allowing both translations and rotations in all coordinate directions, i.e. six degrees of freedom.
In many applications of for example Angle Resolved Photoelectron Spectroscopy (ARPES) a complete measurement requires full detection of a solid angle with a total cone opening of 30 degrees from a well aligned sample. Depending on sample and excitation energy/kinetic energy the required angular range may vary. The angle resolution requirements also varies with application but typically range from 1 degree down to better than 0.1 degrees. In energy resolution the desired span is from 0.5 eV down to 0.5 meV depending on application. In order to achieve a high resolution measurement the analyser arrangement must have sufficient angular and energy resolution, but since the hemispherical analyser arrangement only accepts electrons emitted within a limited angular range perpendicular to the lens axis, the sample manipulator must have very high precision movements and repeatability. The manipulator is needed to accurately rotate and tilt the sample to build up the complete 30 degree solid angle data set.
However, in recent years the illumination systems have reached a much higher level of spatial resolution which means that extremely minute crystallites can be observed. Thereby the manipulation, i.e. rotation of the sample becomes very difficult.
One way of eliminating the sample manipulation is to provide a second deflector inside the lens and close to the first deflector in order to bring the electron beam at the entrance to the measurement region in alignment with the optical axis of the lens.
Spectrometers provided with such deflectors inside the lens have been sold by VG Scienta AB.
Despite the fact that this system eliminated the need for sample manipulation, it still suffers from some distortion in the recorded images.